1. Field of the Invention
The present invention relates to a discharge lamp starting device for starting a discharge lamp and for controlling lighting.
2. Description of the Prior Art
A practical starting circuit for a discharge lamp such as a metal halide lamp has been available for many years. R. T. Elms, J. C. Engel and G. F. Saletta (Journal of IES, 1975 October, pp. 26-30) analyzed the restriking process and found that the restriking process consists of three relatively independent stages: initial breakdown, hot spot formation and arc establishment.
In order to reduce or substantially remove the harmful electrical migration and sonic resonance which occur generally during the starting of a discharge lamp, a known starting device for a discharge lamp supplies a high electrical voltage generated by an LC resonance to the discharge lamp between a pair of electrodes thereof to excite an excitable component in the discharge lamp, and then supplies a high frequency electrical current having an amplitude within a predetermined range and having a predetermined repetition rate to the pair of electrodes using a high frequency inverter, and then periodically changes the direction of the supplied high frequency current.
For example, FIG. 1 shows such a starting circuit for a discharge lamp disclosed in Japanese laid open Patent Publication No. 273,183/1986. In order to start and light a discharge lamp, a high voltage (resonance voltage) generated by the series resonance of an LC circuit consisting of a capacitor C2 and a choke coil L is supplied to a discharge lamp R. The resonance voltage enables a starting energy larger than an energy generated by applying a high voltage pulse to be supplied to the discharge lamp R. Therefore, the starting characteristic can be improved generally when compared with the technique of applying high voltage pulses. However, in order to start (restrike) a discharge lamp of this kind, a breakdown voltage of a few kilovolts to tens of kilovolts is needed. In order to secure such a high voltage using the series resonance of the LC circuit, it is necessary to supply a resonance current of a few amperes to the capacitor C2 and the choke coil L at the same time. Therefore, the core size of the choke coil L has to be increased in order to prevent the saturation of the choke coil L and the sizes of the choke coil L and the capacitor C2 have to be increased in order to secure the insulation against the high voltage generated at a small size resonance. Thus, the starting device cannot easily be manufactured.